This invention relates to an arrangement comprising an input terminal for receiving an electric input signal, which terminal is coupled to an input of a phase-locked loop having an output coupled to an input of a hold circuit having an output coupled to an output terminal of the arrangement for supplying an electric output signal. The phase-locked loop comprises a phase comparator having a first input coupled to the input of the phase-locked loop, a second input and an output, a low-pass filter having an input and an output coupled to the output of the phase comparator and to the output of the phase-locked loop, respectively, a voltage-controlled oscillator having an input and an output coupled to the output of the low-pass filter and to the second input of the phase comparator, The arrangement further comprises detection means for detecting an instantaneous interference in the input signal of the phase-locked loop and for subsequently generating a control signal and for applying said control signal to an output which is coupled to a control input of the hold circuit. The hold circuit is adapted to retain the signal applied to its input under the influence of the control signal.
An arrangement of this type may be incorporated, for example, in a video recorder for demodulating, after reading a registration carrier, an audio signal which is written in FM-modulated form into the registration carrier, for which purpose the arrangement further comprises:
two or more reading heads provided on a rotatable head drum,
switching means having two or more inputs, an output and a control input, which switching means are adapted to couple one of the inputs to the output under the influence of a control signal to be applied to the control input, an output of each of the two or more reading heads being coupled to an associated input of the two or more inputs of the switching means and the output of the switching means being coupled to an input of
a limiter which is coupled between the input terminal of the arrangement and an input of the detection means.
The arrangement is intended to compensate for instantaneous interferences in the input signal of the phase-locked loop. This input signal is an FM-modulated signal in its use in video recorders. The instantaneous interferences are those interferences affecting the regularity in the aforementioned signal. These interferences may be, for example, drop-outs or drop-ins in the input signal of the phase-locked loop or sudden phase changes in this input signal which are produced as a result of switching over from reading one registration carrier with the one reading head to reading with another reading head. These instantaneous interferences result in the phase-locked loop being deregulated so that the voltage-controlled oscillator in the phase-locked loop (hereinafter the phase-locked loop will be referred to as "PLL") can no longer (satisfactorily) follow the input signal of the PLL. This in turn results in (audible) interferences occurring in the PLL-demodulated audio signal.
It is known to energize the hold circuit during the head switching operation so that the audio signal, which was present just before the head switching operation at the output of the PLL, is retained. This is a satisfactory solution for low-frequency audio signals. In the case of high-frequency audio signals, however, a continuity error may occur which may be very disturbing. Moreover, the purely periodic character of energizing the hold circuit at the frequency of the head switching operation produces an additional interference in the output signal of the arrangement.